1. Field of the Invention
The present invention relates to a valve operating system of an internal combustion engine, and in particular to a valve operating system including operating cams and swing cams which cooperate open and close valves.
2. Description of the Related Art
Various types of internal combustion engine valve operating systems have been proposed. The valve operating system of an engine is generally provided with driving or operating cams and rocker arms which are arranged so that the valves of the engine are opened and closed by the operating cams driving the rocker arms. There is known another type of engine valve operating system which includes operating cams and swing cams adapted so that the valves of the engine are opened and closed by the operating cams directly or through swing levers swing the swing cams, as shown in, for example, Japanese Patent Laid-Open No. 55-137306.
In the valve operating system having the operating cams and the swing cams, the location of the cam follower of the swing cam at the time of maximum valve lift becomes increasingly distant from the rotating center of the operating cam as the swing angle of the swing cam becomes larger. The cam nose of the operating cam therefore needs to be large so as to follow the swing motion of the swing cam and, as a result, the valve operating system of the engine becomes large. This is one of the problems of the conventional engine valve operating system.
Therefore, a need has been felt for an engine valve operating system of relatively small size especially for use in automobile engines.
On the other hand, there is known an engine valve timing control system which includes operating cams and swing cams. Japanese Patent Publication No. 58-38602 discloses a valve timing control system of this type wherein both the operating cams and the swing cams are tapered and the valve timing is varied by moving the operating and swing cams relative to each other in the axial direction.
It is known that the volumetric efficiency of an engine can be improved and a higher engine output can be obtained by increasing the positive acceleration of the valve lift. This is because the integrated crank angle of the valve lift, namely the time period area of valve opening, becomes larger for the same opening angle and the same valve lift when the positive acceleration of the valve lift is increased.
In the above-mentioned engine valve timing control system, the valve lift acceleration is the sum of the acceleration component generated by the operating cam and the acceleration component generated by the swing cam. Being the sum of the acceleration of the operating and swing cams, the valve lift acceleration is easily varied by changes in the valve timing. If the valve lift acceleration becomes smaller than that before the valve timing was changed, the time period area of the valve opening was decreased. As a result, the engine output is decreased as the time period is decreased.